Search for Coherent Elastic Scattering of Solar ${}^8\mathrm{B}$ Neutrinos in the XENON1T Dark Matter Experiment

We report on a search for nuclear recoil signals from solar ${}^8\mathrm{B}$ neutrinos elastically scattering off xenon nuclei in XENON1T data, lowering the energy threshold from 2.6 to $1.6\mathrm{keV}$. We develop a variety of novel techniques to limit the resulting increase in backgrounds near the threshold. No significant ${}^8\mathrm{B}$ neutrinolike excess is found in an exposure of $0.6\mathrm{t}\times \mathrm{y}$. For the first time, we use the nondetection of solar neutrinos to constrain the light yield from 1–2 keV nuclear recoils in liquid xenon, as well as nonstandard neutrino-quark interactions. Finally, we improve upon world-leading constraints on dark matter-nucleus interactions for dark matter masses between 3 and $11\mathrm{GeV}{c}^{-2}$ by as much as an order of magnitude.

Figure 1

Top: Improvement of the NR acceptance in this work (solid) with respect to previous DM analyses (dashed) [8, 18], including S1 detection efficiency (blue), software trigger and S2 threshold acceptance (green), and total acceptance after other quality and background rejection cuts (black). The right axis shows the recoil spectrum of ${}^8\mathrm{B}$ $\mathrm{CE}\nu \mathrm{NS}$ or dark matter of mass $6\mathrm{GeV}{c}^{-2}$ and cross section $4\times {10}^{-45}{\mathrm{cm}}^2$ (dotted pink), and the products of this spectrum with the total acceptances (red) as a function of true recoil energy. The acceptances and resulting spectra are based on the nominal (NEST) yield models. The red shaded interval contains 68% of expected CEvNS events. Middle: The most precise available measurements of $Q_y$ [19] (orange), with the $Q_y$ model described in the text overlaid (black). Bottom: Constraints on $L_y$ (in photons per keV) from LUX (orange) [20], and the 68% upper limit from this work described in the Results section (blue), with the $L_y$ model described in the text overlaid (black). To be conservative, no response is assumed below the 0.5 keV cutoff (hatched gray).

Figure 2

Events in the science dataset (pink circles) and the AC-enriched validation region (blue crosses) projected onto $Z$ and the quantile of the ${\mathrm{S}2}_{\mathrm{prev}}/\mathrm{\Delta }{t}_{\mathrm{prev}}$ value for NR signals. The AC model is shown in gray. Smaller panels show the projection of the model and data onto each axis, as well as the ${}^8\mathrm{B}$ $\mathrm{CE}\nu \mathrm{NS}$ model (green dashed), normalized to its upper limit. The AC-enriched region data in blue has a slightly different $Z$ distribution due to the inverted GBDT cut, but is included for illustration, scaled by 0.36, the ratio of expected AC events in each dataset.

Figure 3

Projections of the 90% confidence volumes in $L_y$ and $\mathrm{\Phi }$ (top), and in $L_y$ and the $Q_y$ interpolation parameter $q$ (bottom). The green area shows constraints using only the XENON1T data. Combining the XENON1T data and external constraints on $Q_y$ [19] and $L_y$ [22, 37] (shown in black dash-dotted lines) gives the confidence interval shown in pink, and an upper limit on $\mathrm{\Phi }$. Conversely, combining the XENON1T data and constraints on $\mathrm{\Phi }$ [16] and $Q_y$ yields the dark blue interval and upper limits on $L_y$. The dashed white line displays the 68% confidence interval. $L_y$ is assumed constant in the ${}^8\mathrm{B}$ $\mathrm{CE}\nu \mathrm{NS}$ ROI for these constraints.

Figure 4

Constraints on new physics using XENON1T data. Top: Constraints on nonstandard vector couplings between the electron neutrino and quarks, where the XENON1T 90% confidence interval (light blue region) is compared with the results from COHERENT [3, 34] (pink and dark red regions) and CHARM [38] (green). Bottom: The 90% upper limit (blue line) on the spin-independent DM-nucleon cross section ${\sigma }_{\mathrm{SI}}$ as function of DM mass. Dark and light blue areas show the $1\sigma$ and $2\sigma$ sensitivity bands, and the dashed line the median sensitivity. Green lines show other XENON1T limits on ${\sigma }_{\mathrm{SI}}$ using the threefold tight-coincidence requirement [8] and an analysis using only the ionization signal [9], and other constraints [39, 40, 41, 42, 43, 44] are shown in red. The dash-dotted line shows where the probability of a $3\sigma$ DM discovery is 90% for an idealized, extremely low-threshold (3 eV) xenon detector with a $1000\mathrm{t}\times \mathrm{y}$ exposure [45]. The black dot denotes DM that has a recoil spectrum and rate identical to the ${}^8\mathrm{B}$ neutrinos.